1. Field of the Invention
The present invention relates to an apparatus for controlling a multi-cylinder internal combustion engine with partial cylinder switch-off mechanism. The engine is capable of regulating the number of cylinders in operation between the following two modes, one being a mode in which all cylinders are operated (hereinafter called "all-cylinder operation"), the other being a mode in which some cylinders are operated while the other cylinders (i.e., partial cylinders) are suspended in operation by switching them off (hereinafter called "partial-cylinder operation").
2. Description of the Related Art
This type of conventional multi-cylinder internal combustion engine is provided with hydraulically operated valve switching means on an intake side and on an exhaust side, respectively, to switch intake valves and exhaust valves for part of cylinders between a driving state and a state in which the driving of the valves is stopped or suspended (also called a "drive-free state"). At the time of partial-cylinder operation, the intake valves and the exhaust valves of part of the cylinders are switched to the drive-free state.
In order to avoid a loss in torque due to compression of intake air at a compression stroke or at an exhaust stroke in cylinders not in operation (hereinafter also called "suspended cylinders"), the driving of inlet valves must be stopped ahead of, or prior to, exhaust valves at the time of switching from the all-cylinder operation to the partial-cylinder operation. On the other hand, in order to prevent oil from flowing into the suspended cylinders, the driving of the exhaust valves must be stopped ahead of the intake valves at the time of switching from the all-cylinder operation to the partial-cylinder operation. Further, in order to avoid a deviation in an air/fuel ratio at the time of returning back to the all-cylinder operation, due to gas which may remain in the suspended cylinders, the exhaust valves must be returned to driving ahead of the intake valves at the time of switching from the partial-cylinder operation to the all-cylinder operation.
There is known an apparatus as described hereinbelow (see the item entitled "Problems that the Invention is to Solve" in Japanese Published Unexamined Patent Application No. 74545/1996). Namely, in this conventional apparatus, in order to stop the driving of, or to resume the driving of, one of the intake valves and the exhaust valves ahead of the other thereof at the time of switching between the all-cylinder operation and the partial-cylinder operation, there is separately provided a control valve for switching the input hydraulic pressure to each of valve switching means on the intake side and on the exhaust side, respectively. At the time of switching of operation, the control of switching of the hydraulic pressure by the control valve on the intake side and the control of switching of the hydraulic pressure by the control valve on the exhaust side are performed at a predetermined time lag.
If the control valve on the intake side and the control valve on the exhaust side are controlled at a time lag as in the above-described conventional example, and at the time, e.g., of switching from the all-cylinder operation to the partial-cylinder operation, if the switching control of the hydraulic pressure by the control valve on the exhaust side is performed at a predetermined time after the switching control of the hydraulic pressure by the control valve on the intake side has been finished, there is the following possibility. Namely, in case the operation of a plurality of cylinders is suspended, there is a possibility, depending on a rotational frequency of the engine, that the switching control of the hydraulic pressure is performed by the control valve on the intake side during the compression stroke of any one of the cylinders and that the switching control of the hydraulic pressure by the exhaust valves is performed during the ignition stroke. According to this arrangement, the exhaust valves will no longer be opened in the exhaust stroke and, consequently, the driving of the exhaust valves will substantially be stopped ahead of the intake valves. Therefore, in the example of the above-described conventional apparatus, there is no guarantee that the driving of the intake valves and the exhaust valves in all of the suspended cylinders is stopped or resumed in a given order.
In view of the above-described points, the present invention has an object of providing an apparatus for controlling a multi-cylinder internal combustion engine, in which it is possible to stop the driving of, or to resume the driving of, the intake valves and the exhaust valves for all of the suspended cylinders in a given order irrespective of the rotational frequency of the engine.